Coating thickness measurement gauges of the magnetic type are used for measuring, in a non-destructive manner, the thickness of coatings placed on a ferromagnetic substrate. The operation of those gauges is based on the principle that the magnitude of the force of magnetic attraction between a ferromagnetic substrate and a magnet will vary depending upon the thickness of the coating which separates the magnet from the ferromagnetic substrate. The greater the thickness of the coating, the lesser the force of magnetic attraction.
Gauges of this type are used for measuring the thickness of coatings in many different coating processes such as, for example, plating, painting and enameling, metal spraying, welding, boiler-making and other areas where protective coatings are applied to an article. The magnetic type gauges are advantageous in that they permit the thickness of a coating to be quickly and easily determined in a non-destructive manner. Although the magnetic type gauges currently available are able to provide relatively accurate results in many instances, they do suffer from certain drawbacks.
One of these drawbacks relates to their susceptibility to drifting when used to measure the thickness of a coating on a substrate whose temperature is above or below room temperature, such as in the case of freshly hot dipped galvanized objects or powder coated articles emerging from melting ovens. Known coating thickness measurement gauges utilize rare earth alloy magnets of the SmCo.sub.5 and Sm.sub.2 Co.sub.17 type. It has been discovered that errors in coating thickness measurement on the order of four percent per 100.degree. C. above or below room temperature can result when using such gauges which employ only light rare earth elements in combination with a transition element. The errors arise because the magnetic strength of the light rare earth magnet alloys varies with changes in temperature.
Since it is oftentimes necessary to determine the thickness of the coating as soon as possible after the coating has been applied, waiting until the substrate has reached room temperature is not a desirable alternative.
Accordingly, the need exists for a coating thickness measurement gauge that is able to provide accurate coating thickness measurements over a wide range of temperatures. Similarly, there exists a need for a gauge that permits the thickness of coatings to be measured immediately after the coating has been applied.
Another problem that arises concerns the inability of known coating thickness measurement gauges of the magnetic type to compensate for the affect that gravity has on the accuracy of the measurements obtained. For example, U.S. Pat. No. 4,392,305 discloses a gauge which employs a permanent magnet suspended on a coil spring within a housing. When such a gauge is utilized to measure the thickness of a coating on a vertical wall or some other vertically oriented member, gravity has little or no effect upon the operation of the gauge because the magnetic attraction force between the magnet and the ferromagnetic substrate is perpendicular to the force of gravity. However, when measuring the thickness of a coating on a horizontally oriented member, such as a coated member located on the ground or extending across the ceiling, the force of gravity acts parallel to the force of magnetic attraction, thereby giving inaccurate results. For example, when measuring the thickness of a coating on a floor, the force of gravity acts in the same direction as the force of gravity and consequently, the measured thickness of the coating will be less than the actual coating thickness.
There exists a need, therefore, for a coating thickness measurement gauge of the magnetic type that is able to compensate for the affect of gravity in order to produce accurate and reliable coating thickness measurements.